Protein A is a protein which constitutes five percent of the cell wall of staphylococcus aureus and is abbreviated as “SpA”. The Protein A is consisting of five domains of A to E domains and doesn't contain cysteine.
A protein consisting of the Protein A or at least one domain of the A to E domains (hereinafter, “SpA protein”) has a property of being able to bind to an antibody. Utilizing this property, the Spa Protein is used to bind the antibody to a substrate.
According to a prior art, a lysine residue contained in the Spa Protein is bound to the carboxyl group on the substrate with use of amine-coupling method.